Chemical Measurements

We can show what’s happening in a reaction by writing chemical equations. It’s important that symbol equations are balanced so there are equal numbers of atoms on each side of the reaction. This is because no atoms are lost or created during a chemical reaction, which means that mass is conserved before and after the reaction.

Balancing equations

The law of conservation of mass states that no atoms are made or lost during a reaction. This means that the number of atoms of each element in our reactants is equal to the number of atoms of each element in our products. It also means that the mass of the reactants will be equal to the mass of our products.

Let’s have a look at how to write a symbol equation for the reaction between magnesium iodide and chlorine. This is a displacement reaction which forms magnesium chloride and iodine.

To start with, we need to figure out the formula of magnesium iodide. This can be done by looking up their symbols on the periodic table: Mg for magnesium and I for iodine. When these elements come together to form a molecule, it may not be the case that one magnesium atom bonds with only one iodine atom. Instead, we have to pay attention to the charges on the ions formed by these atoms. Magnesium forms an ion with a +2 charge, since it needs to lose 2 outer electrons to get a full outer shell. Iodide ions have a +1 charge, since iodine needs to gain 1 more electron to get a full outer shell. For the whole molecule to be unbalanced, we need two iodide ions to balance out the +2 charge on the magnesium ion. This gives us a formula of MgI₂. Remember that when we are writing the formula of a molecule, we multiply atoms by writing numbers after the atom we want to multiply in subscript. You can also use the crossing-over method to work out the formula of a molecule if you know the charges of the ions.

To write the rest of our symbol equation, we need to remember that anything in group 7 (along with hydrogen, nitrogen and oxygen) is diatomic. This means that when it is hanging out on its own, it exists as molecules made up of 2 atoms so we need to write a little number 2 after it. Chlorine is written Cl₂ and iodine is written I₂. For the formula of magnesium chloride, we can use our crossing-over method again. Magnesium ions have a +2 charge and chloride ions have a -1 charge. This means that we multiply the magnesium by 1 and the chlorines by 2, giving us MgCl₂.

Now we need to check if our equation is balanced. On each side of my equation I have 1 magnesium, 2 chlorines and 2 lots of iodine so I don’t need to add any big numbers in front of the molecules to balance them out.

Let’s have a look at an equation which we do need to balance. Look at the equation below showing the reaction between hydrogen and nitrogen to form ammonia.

On the left hand side of the equation we have 2 hydrogen atoms but 3 on the right. We also have an unbalanced number of nitrogen atoms – 2 on the left and 1 on the right. We can start balancing the nitrogens by putting a big number 2 in front of the ammonia molecule. This changes the number of hydrogens on the right hand side to 6, which means that we can put a big number 3 in front of the hydrogen molecule on the left to get equal numbers on both sides.

Relative formula mass

The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

For example, the Mr of calcium carbonate, CaCO₃, is the sum of the atomic masses of all of the elements in the compound. The mass number of calcium is 40, carbon is 12 and oxygen is 16, so the Mr for calcium carbonate is 40 + 12 + (16 x 3) = 100.

If you have a balanced chemical equation, the sum of the formula masses (Mr) of the reactants is equal to the sum of the formula masses (Mr) of the products. Look at the equation below for the decomposition of calcium carbonate into calcium oxide and carbon dioxide.

In this reaction, calcium carbonate is the only reactant and we have already calculated its Mr as 100. Since we know that the Mr of the reactants is equal to the Mr of the products, this means that the formula masses of calcium oxide and carbon dioxide will add to 100. This is shown below:

Formula mass of calcium oxide = 40 + 16 = 56
Formula mass of carbon dioxide = 12 + (16 x 2) = 44
Total formula mass of products = 56 + 44 = 100

You may be asked to calculate the percentage by mass in a compound – this indicates how much weight a particular element is contributing to the whole molecule. For example, let’s say we need to find out the percentage by mass of carbon in carbon dioxide. All we need to do is divide the atomic mass of carbon by the formula mass of carbon dioxide. In this case, that’s 12 divided by 44. We then multiply by 100 to convert it into a percentage.

12/44 = 0.27
0.27 x 100 = 27%

This means that 27% of the mass of the whole molecule comes from carbon. This means that the remaining 73% will come from the two atoms of oxygen.

Change of mass when the reactant or product is a gas

We’ve learnt that in a balanced equation, the formula masses (Mr) of the reactants will always be equal to the formula masses of the products. Since we also have the same number of moles of products and reactants, this means that the mass of the products will always be equal to the mass of the reactants. Let’s think back to the thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide. If I heat 20 g of calcium carbonate, this will decompose into calcium oxide and carbon dioxide with a total weight of 20 g.

However, if we were conducting this experiment in the lab with the calcium carbonate in a flask on a weighing balance, it may appear that there has been a change in mass during the reaction. This can be explained by the fact that one of the products (carbon dioxide) is a gas and has escaped from the flask. This would result in the calcium carbonate appearing to lose mass as the reaction progresses.

Alternatively, other reactions may appear to involve an increase in mass. This time, the reactant will be a gas whose mass hasn’t been accounted for. For example, if I have 5 g of iron which reacts with the oxygen in the air, the rust (iron oxide) that is formed will have a larger mass than the 5 g of iron that we started with.

Uncertainty in measurements

Whenever a measurement is made there is always some uncertainty about the result obtained. Let’s say you need to measure out 100 cm³ of water for an experiment, it’s difficult to say whether you have actually measured 100 cm³ or 101.3 cm³ or 98.59 cm³ and so on. The more precise the measuring instrument (i.e. the smaller the gradations), the more certain we can be about the measurement. For experiments involving repeats, the larger the range in the data (i.e. the more they deviate from the mean), the larger the amount of uncertainty.

Did you know…

The school lab isn’t the only place you’ll find calcium carbonate. You’ll find it in chalky cliffs, rocks and eggshells. Some fish even have floating calcium carbonate deposits in their heads called ‘earstones’, which are pulled down by gravity. On land, plants have starch grains in their root tips, which sink toward the ground, helping to guide their roots downward. The calcium carbonate in these organisms helps them to sense the direction of the Earth’s gravitational pull.

Next Page: Mass and Mole Calculations